Method for separating a C4-hydrocarbon mixture

ABSTRACT

A C 4 -hydrocarbon mixture essentially containing 1,3-butadiene, butenes, butanes and other C 4 -hydrocarbons is separated into at least 4 fractions, 
     a) the fraction (a) essentially comprising 1,3-butadiene, 
     b) the fraction (b) essentially comprising butenes, 
     c) the fraction (c) essentially comprising butanes and 
     d) one or more fractions (d) essentially comprising 1,3-butadiene and the other C 4 -hydrocarbons, 
     by extractive distillation by means of N-methyl-2-pyrrolidinone or an aqueous solution of N-methyl-2-pyrrolidinone (NMP).

The present invention relates to a process for separating aC₄-hydrocarbon mixture essentially containing 1,3-butadiene, butenes,butanes and other C₄-hydrocarbons into at least 4 fractions,

a) the fraction (a) essentially comprising 1,3-butadiene,

b) the fraction (b) essentially comprising butenes,

c) the fraction (c) essentially comprising butanes and

d) one or more fractions (d) essentially comprising [lacuna] the otherC₄-hydrocarbons,

 by extractive distillation by means of N-methyl-2-pyrrolidinone or anaqueous solution of N-methyl-2-pyrrolidinone (NMP),

 wherein

1. the gaseous C₄-hydrocarbon mixture is first brought into contact withNMP in an extraction zone (I), the 1,3-butadiene and the otherC4-hydrocarbons being essentially completely absorbed by the NMP but thebutenes and butanes remaining essentially in the gas phase;

2. the unabsorbed butenes and butanes (gas stream bc) and the extractionsolution formed in step 1 (extraction solution ad) are removed from theextraction zone (I);

3. the extraction solution (ad) is transferred to a desorption zone (I)at a lower pressure and/or higher temperature than the extraction zone(I) and 1,3-butadiene is desorbed from the extraction solution (ad), themain part of the other C₄-hydrocarbons remaining in the liquid phase;

4. the extraction solution formed in stage 3 (extraction solution d) andthe desorbed 1,3-butadiene (fraction a) are removed separately from thedesorption zone (I) and, if required, a part of the fraction (a) isreturned to the extraction zone I;

5. the extraction solution (d) is transferred to a second desorptionzone (II) at a lower pressure and/or higher temperature than thedesorption zone (I) and having a pressure and/or temperature gradient,and the other C₄-hydrocarbons and the 1,3-butadiene still remainingtherein are fractionally desorbed from the extraction solution (d) as atleast two separate fractions (d), with the content of the otherC₄-hydrocarbons being at least 10 times higher in at least one of thefractions (fractions d) than in the extraction solution (d), based onthe content of all C₄-hydrocarbons, and the content of the otherC₄-hydrocarbons being correspondingly lower in at least one of thefractions (fractions dR) than in the fractions (d), based on the contentof all C₄-hydrocarbons,

6. the NMP, formed in the desorption zone (II) and essentially free ofC₄-hydrocarbons, and the fractions (d) and (dR) are removed separatelyfrom the desorption zone II, and one or more fractions (dR) are returnedto the desorption zone (I),

7. the gas stream (bc) is first brought into contact with the NMP formedin step 6 in an extraction zone (II), the butenes being essentiallycompletely absorbed by the NMP but the butanes remaining essentially inthe gas phase;

8. the unabsorbed butanes (fraction c) and the extraction solutionformed in step 1 (extraction solution b) are removed from the extractionzone (II);

9. the extraction solution (b) is transferred to a desorption zone (III)at a lower pressure and/or higher temperature than the extraction zone(II) and the butenes are desorbed from the extraction solution (b);

10. the NMP, formed in step 9 and essentially free of C₄-hydrocarbons,and the desorbed butenes (fraction b) are removed from the desorptionzone (III);

11. the NMP formed in step 9 is recycled to one of the extraction zones.

This process is shown schematically in FIG. 1.

A process for separating 1,3-butadiene from a C₄-hydrocarbon mixture isdisclosed, for example, in DE-A-2724365. Briefly, a butane/butene mixedfraction, a 1,3-butadiene fraction and a fraction which contains theother C₄-hydrocarbons are obtained in this process from a C₄-hydrocarbonmixture which contains butanes, butenes, 1,3-butadiene and otherC₄-hydrocarbons, by extractive distillation with NMP as absorbent overvarious absorption and desorption stages. In the entire process, the NMPrequired passes through a closed circulation. NMP which no longercontains any C₄-hydrocarbons (unladen NMP) is first laden with theC₄-hydrocarbon mixture at the beginning of a cycle, passes through thevarious absorption and desorption stages until, at the end of a cycle,unladen NMP is provided by completely desorbing the C₄-hydrocarbons. Theprocess is distinguished by the fact that the individual stages areparticularly advantageously coupled via indirect heat exchangeprocesses.

The separation of 1,3-butadiene and 2-butenes into separate fractionsand the separation of 1,3-butadiene and acetylenes into separatefractions from C₄-hydrocarbon mixtures have been described by V. A.Gorshkov et al. in the publication The Soviet Chemical Industry, No. 11,November 1971.

EP-A-141356 likewise relates to the separation of a 1,3-butadienefraction from a C₄-hydrocarbon mixture by means of extractivedistillation using NMP. The use of columns in which absorption anddesorption zone are integrated in a single column in each case make thisprocess particularly economical.

EP-A-5788 discloses a process for separating a 1,3-butadiene fractionand a butyne fraction from a C₄-hydrocarbon mixture by means ofextractive distillation using NMP.

EP-A-9630 relates to a process for separately removing styrene and1,3-butadiene from a mixture which otherwise contains C₄-hydrocarbons,the styrene first being separated from the mixture by distillation and1,3-butadiene being separated from the remaining mixture by means ofextractive distillation.

U.S. Pat. No. 5,242,550 discloses the separation of a butene/butanemixture by means of extractive distillation using NMP as absorbent.

It is an object of the present invention to provide a process whichpermits the separation of a C₄-hydrocarbon mixture into a butanefraction, butene fraction, 1,3-butadiene fraction and a fraction whichcontains the other C₄-hydrocarbons in a particularly efficient andeconomical manner. In particular, the required quantities of energy andthe capital costs should be particularly low in this process.

We have found that this object is achieved by the process described atthe outset.

The process can be applied to C₄-hydrocarbon mixtures which contain1,3-butadiene, butenes, butanes and other C₄-hydrocarbons plus verysmall amounts of C₃- and C₅-hydrocarbon impurities.

Such C₄-hydrocarbon mixtures are obtained, for example, as C₄ fractionsin the production of ethylene and/or propylene by thermal cleavage of apetroleum fraction, for example of liquefied petroleum gas (LPG),naphtha, gas oil or the like as hydrocarbon fraction. Furthermore, suchC₄ fractions are obtained in the catalytic dehydrogenation of n-butaneand/or n-butene. The C₄ fractions obtain [sic], as a rule, butanes,n-butene, isobutene, 1,2 butadiene, vinylacetylene, ethylacetylene and1,2-butadiene [sic] and may contain small amounts of C₅-hydrocarbons,the 1,3-butadiene content being in general from 10 to 80, preferablyfrom 20 to 70, in particular from 30 to 60, percent by weight while thecontent of vinylacetylenes, ethylacetylene and 1,2-butadiene (referredto below as other hydrocarbons) together in the C₄ fractions generallydoes not exceed 5 percent by weight.

The novel process can advantageously be employed in particular to thoseC₄-hydrocarbon mixtures which contain

from 10 to 80% by weight of 1,3-butadiene;

from 10 to 60% by weight of butenes;

from 5 to 40% by weight of butanes;

from 0.1 to 5% by weight of other C₄-hydrocarbons and

from 0 to at most 5% by weight of C₃- and C₅-hydrocarbons.

The n-methyl-2-pyrrolidinone or its aqueous solution employed asselective solvent (N-methyl-2-pyrrolidinone and its aqueous solutionabbreviated to “NMP” for short hereinafter) is generally a conventionalindustrial product which may contain up to 15% by weight of water.

The extraction zones are preferably in the form of columns through whichthe gas streams are passed countercurrently to the NMP.

In step 1, the C₄-hydrocarbon mixture to be separated is first fed ingaseous form with NMP into an extraction zone (I) and brought intocontact with one another there, the 1,3-butadiene and the otherC₄-hydrocarbons being essentially completely absorbed by the NMP but thebutenes and butanes remaining essentially in the gas phase. In the NMPand C₄-hydrocarbon mixture fed in, the ratio of NMP to C₄-hydrocarbonmixture is from 5:1 to 20:1 in the extraction zone (I).

The generally known extraction methods are suitable for this extractionstep.

From the extraction zone (I), in general a gas stream which [lacuna] inparticular unabsorbed butanes and butenes and, if C₃- andC₅-hydrocarbons are present as impurity in the C₄ mixture, also propane,propene and propadiene plus traces of C₅-hydrocarbons (gas stream bc) isremoved at the top of column and the extraction solution (extractionsolution ad) is removed from the bottom of the column.

The extraction solution (ad) contains in general only from 0 to 2% byweight of butenes and butanes, plus, if present, propyne and/or almostthe total amount of C₅-hydrocarbons.

The gas stream (bc) contains, in addition to the butenes and butanes, ingeneral only from 0 to 1% by weight of the 1,3-butadiene originallypresent in the C₄-hydrocarbon mixture and of the other C₄-hydrocarbons.

The extraction zone (I) is generally in the form of a scrubbing columnwith plates, dumped packings or structured packings as internals. Thesepreferably have from 40 to 80 theoretical plates. The column pressuredepends on the temperature of the cooling medium (well water, riverwater, sea water, refrigerants such as liquid propylene, liquid ammoniaor brine). It is between 2 and 6 bar, preferably 4.5 bar. Thetemperature profile in the extraction zone is determined by thetemperature of the NMP. It is advantageous to lower the temperatureprofile by partial condensation of the fraction (bc) because theseparation efficiency is improved at lower temperature. A typical valuefor the condensation is 20%. This results in a temperature of from 40 to60° C. at the top of the column.

For the desorption of the 1,3-butadiene from the extraction solution(ad), the latter is transferred to a desorption zone (I) at a lowerpressure and/or higher temperature than the extraction zone (I) and1,3-butadiene (1,3-butadiene fraction a) is desorbed from the extractionsolution (ad), the main part of the other C₄-hydrocarbons, propyne andC₅-hydrocarbons remaining in the liquid phase.

Preferably, the pressure in the desorption zone (I) is the same as thatin the extraction zone (I) and the temperature is from 20 to 25° C.higher than in the extraction zone (I).

The 1,3-butadiene fraction (a) removed from the desorption zone (I)usually has a purity of from 95 to 99% by weight.

The extraction solution (d) formed by desorption of 1,3-butadiene in thedesorption zone (I) is then removed from the desorption zone (I) andtransferred to a second desorption zone (II) at a lower pressure and/orhigher temperature than the desorption zone (I). During transfer of theextraction solution (d) from desorption zone (I) to (II) itadvantageously passes through a heat exchanger zone in which a part ofthe hydrocarbons in the extraction solution (d) evaporates, and this gasstream is directly fed back into the bottom of desorption zone (I).Pressure and temperature are chosen so that virtually allC₄-hydrocarbons still remaining in the NMP are desorbed; they are ingeneral 1.5 bar and 150° C.

In desorption zone II there is fractional desorption from the extractionsolution (d) of 1,3-butadiene still present therein and of the otherC₄-hydrocarbons plus, where appropriate, [lacuna] and C₅-hydrocarbonsstill present therein as at least two separate fractions (d), with thecontent of other C₄-hydrocarbons being at least 10 times, in generalfrom 10 to 100 times, preferably from 20 to 80 times, higher in at leastone of the fractions (fraction d) than in the extraction solution (d),based on the content of all C₄-hydrocarbons, and the content of theother C₄-hydrocarbons being lower in at least one of the fractions(fractions dR) than in the fractions (d), based on the content of allC₄-hydrocarbons. The hydrocarbons in the extraction solution (d) arepreferably fractionated in the desorption zone (II) into a fraction (d)and a fraction (dR), where fraction (d) preferably comprises essentiallyat least 20% by weight, particularly preferably from 20 to 40% byweight, of other C₄-hydrocarbons and otherwise butadiene, and fraction(dR) comprises essentially more than 80% by weight, particularlypreferably from 85 to 95% by weight, of butadiene and otherwise otherC₄-hydrocarbons.

The NMP formed in the desorption zone (II) and essentially free ofC₄-hydrocarbons, and fractions (d) and (dR) are removed separately fromthe desorption zone II, and one or more of the fractions (dR) arereturned to the desorption zone (I), e.g. to the bottom of the scrubbingcolumn.

The pressure gradient in this case is preferably overcome by means of acompressor. The fraction (d) is normally treated countercurrently withwater (condensate) in order to absorb most of the NMP present therein.

In general, the ratio by weight of the fractions returned to thedesorption zone (I) to those removed from the system is from 20:1 to80:1.

The desorption zone (II) consists in general of a main column with aside column. Both are designed as scrubbing columns. The main columngenerally contains packings because the low pressure drop thereof hasparticularly beneficial effects here. The main column should have from10 to 15 theoretical plates. The side column generally has 10 practicalplates. The pressure is generally from 1.5 to 1.6 bar; the temperatureat the bottom of the main column is from 140 to 150° C. and at the topthereof is from 80 to 100° C. While the fractions (d) are removed assidestream preferably at from 130 to 140° C., the fractions (dR) arenormally taken off overhead.

If a 1,3-butadiene fraction having a particularly high purity isdesired, the following procedure is preferably adopted:

The 1,3-butadiene fraction (a) which is removed from the desorption zone(I) is divided into two part streams of fraction (a1) and (a2), andfraction (a1) is returned to the extraction zone I (is preferably passedto the bottom of the extraction column I) and fraction (a2) is againbrought into contact, in an extraction zone (III), with NMP which wasrecovered from the desorption zone (II) or (III), a part of the fraction(a2) and the predominant part of other C₄-hydrocarbons still containedas impurity in the fraction (a2) being absorbed by the NMP (extractionsolution ax).

The unabsorbed part of the fraction (a2) (fraction a3) is removedseparately from the extraction zone, and the extraction solution (ax) isreturned to the extraction zone (I).

This variant is illustrated in FIG. 2.

The ratio by weight of NMP to 1,3-butadiene fraction (a) generallycorresponds to from 1:3 to 1:7, depending on the composition of theinitial C₄ mixture and the specifications for fraction (a3).

The ratio of the material streams of fractions (a1) and (a2) is normallyfrom 1:1 to 4:1.

The 1,3-butadiene fraction (a3) still contains impurities in particularin the form of C₃- and C₅-hydrocarbons and 1,2-butadiene. Theseimpurities are in general subsequently removed in two conventionaldistillation columns.

Regarding the design of the extraction column and the parameters ofpressure and temperature, the same applies in general terms to theextraction zone (III) as to the extraction zone (I). The ratio of NMPfed in to the crude butadiene fraction (a2) corresponds to from 1:3 to1:7.

The unabsorbed 1,3-butadiene and the 1,3-butadiene taken off from theextraction zone (III) normally has a purity of more than 98% by weight.

The gas stream (bc), optionally with the addition of the external addedstream (gas stream Zbc is first brought into contact, in an extractionzone (II), with the NMP recovered in the desorption zone (II), thebutenes being essentially completely absorbed by the NMP but the butanesremaining essentially in the gas phase.

The extraction zone (II) is in general in the form of a scrubbing columnwith plates, dumped packings or structured packings as internals. Thesemust have from 30 to 70 theoretical plates in order to achieve asufficiently good separation effect. The pressure in the extraction zone(II) is chosen so that the gas stream (bc) is able to pass from theextraction zone (I) without further technical assistance into theextraction zone (II). It also depends on the cooling medium availablefor condensing the fraction (c). A typical value for the pressure is 4.0bar, provided water is used for cooling.

The scrubbing column is advantageously equipped in the top of the columnwith a back-wash zone which comprises, for example, 4 theoreticalplates. This back-wash zone serves for recovering the NMP present in thegas phase by means of back-flow of liquid hydrocarbon, for which purposefraction (c) has previously been condensed. It is possible at the sametime to influence thereby the temperature profile in the extraction zone(III). It also applies in this case, as already mentioned in extractionzone (I), that a lower temperature promotes the separation efficiency.Typical temperatures at the top of the column are between 35 and 45° C.

The ratio by weight of NMP to gas stream (bc), including gas stream Zbcwhere appropriate, in the feed to extraction zone (II) is from 10:1 to20:1, depending on the specifications for fractions (b) and (c) and thecomposition of the initial C₄ mixture and of the added stream Zbc.

In the extraction zone (II), a gaseous butane fraction (fraction c) andan extraction solution (b) containing the butene fraction (fraction b)are formed. If the extractive distillation is carried out as describedabove, a fraction (b) which is contaminated with up to 5% by weight ofbutanes and a fraction (c) which is contaminated with up to 15% byweight of butenes are obtained.

The extraction solution (b) is transferred to a desorption zone (III) ata lower pressure and/or higher temperature than the extraction zone(II), the butenes being desorbed from the extraction solution (b). Thedesorption of the butenes and of any other C₄-hydrocarbons containedtherein as impurity can in principle be carried out similarly to thedesorption of the other C₄-hydrocarbons in the desorption zone (II).

The desorption zone (III) may be, for example, in the form of ascrubbing column which has from 5 to 15, preferably from 8 to 10,theoretical plates and a back-wash zone with, for example, 4 theoreticalplates. This back-wash zone serves for recovering the NMP present in thegas phase by means of a back-flow of liquid hydrocarbon, for whichpurpose the fraction (b) has previously been condensed. It isadvantageous to provide packing beds as internals. The pressure at thetop of the column is generally 1.5 and [sic] 1.6 bar. The temperature inthe bottom of the column is generally from 130 to 150° C.

The NMP recovered in the desorption zone (III) is returned to theextraction zones (I), (II) and/or (III).

An additional advantage accrues when the NMP recovered in the desorptionzone (III) is fed back only into the extraction zones (I) and (II), andthe NMP recovered in the desorption zone (II) is essentially fed backinto the extraction zone (III).

The advantage derives from the fact that the removal of butenes/butanesfrom a C₄-hydrocarbon mixture using NMP takes place more easily than theseparation of a mixture of butenes and butanes into two high-puritybutene and butane fractions. Moreover a single solvent circulation ismaintained.

In contrast to the extraction zone (III), which requires high-puritydegassed NMP, the quality of the NMP for the extraction zones (I) and(II) does not need to be so high. This signifies a gain economically inthat the degree of degassing of the NMP, and thus the consumption ofexternal steam for desorbing hydrocarbons in the desorption zone (III),does not need to be so high. In contrast to the NMP from the desorptionzone (II), where from 0 to 10 ppm by weight of C₄-hydrocarbons aredesired, it is perfectly possible for the NMP from the desorption zone(III) to have 1000 or more ppm by weight. This does not impair thepurity of product fractions (b) and (c). On the other hand, however, acontent of hydrocarbons reduces the boiling point of the solvent. Sincethe heat content of the NMP from the desorption zone (III) is utilized,however, for reasons of economy, it is not possible to continue reducingthe boiling point by increasing the residual content of hydrocarbonsindefinitely. The bottom temperatures of from 130 to 150° C. indicatedpreviously result for these reasons. At a bottom temperature of 138° C.,the resulting residual content of hydrocarbons is about 800 ppm byweight.

The novel process can be carried out particularly economically if theheat of the NMP which is obtained by boiling up the extraction solutions(b) and (d) is fed to the desorption zone (I), (II) and/or (III) byindirect heat exchange in a heat exchange zone, and the desorption iseffected in these desorption zones by increasing the temperature in thedesorption zone (I) relative to that in the extraction zone (I), andincreasing the temperature in the desorption zone (II) relative to thatin the desorption zone (I) and increasing the temperature in thedesorption zone (III) relative to that in the extraction zone (II).

The separation of the fraction (a) (butadiene) from the C₄-hydrocarbonmixture is preferably carried out as described in DE-A-2724365. Thispart of the process is particularly preferably carried out as describedin FIG. 3.

According to this process variant, the following procedure is adopted:

The extractive distillation is carried out in more than one column, ingeneral in two columns which together have more than 100 practicaltrays. When using two columns, the absorption stage situated above thepoint at which the C₄-hydrocarbon mixture is fed into the extractivedistillation zone is advantageously located in the first column and theconcentration stage situated below the feed point of the hydrocarbonmixture is advantageously located in the second column, i.e. the feedpoint for the hydrocarbon mixture is at the top of the second column orpreferably at the bottom of the first column. Preferably, no compressionstage is located between absorption stage and concentration stage, andinstead the pressure conditions maintained within the extractivedistillation zone are those automatically established in the extractivedistillation zone in the absence of compression and/or pressurereduction stages within the extractive distillation zone, so that thepressure at the bottom of the extractive distillation zone correspondsat least to the pressure at the top of the extractive distillation zone,in line with the usual pressure loss. As a rule, the pressure differencebetween top and bottom of the extractive distillation zone is from 0.1to 3, preferably from 0.2 to 2, bar.

In general, pressures of from 1 to 9, preferably from 2 to 8, inparticular from 3 to 7, bar are used in the extractive distillationzone. The pressures in the lower third of the extractive distillationzone, i.e. in the region which is occupied by the lower trays of theextractive distillation zone, which correspond to about a third of thetotal number of trays of the extractive distillation zone, are as a rulefrom 1.5 to 9, preferably from 2.5 to 8, in particular from 3.5 to 7,bar.

The extract taken off from the extractive distillation zones is firstbrought to a higher pressure than the pressure in the extractiondistillation zone.

This can be effected, for example, by means of a liquid pump. Ingeneral, this pressure increase is effected essentially isothermally,i.e. the only temperature changes which occur, for example a temperatureincrease up to 1 °C., are those which are caused by the measure leadingto the pressure increase, for example the pumping process. In general,the extract is brought to pressures which are from 1 to 20, preferablyfrom 2 to 18, in particular from 3 to 15, bar above the pressure in theextractive distillation zone, in particular above the pressure in thelower third of the extractive distillation zone.

The extract under increased pressure is then heated in a heat exchangezone by indirect heat exchange with the selective solvent obtained as abottom product from the solvent recovery zone. The selective solvent isrecycled to the extractive distillation zone after the heat exchange. Asa result of the heat exchange with the selective solvent, thetemperature of the extract is generally increased by from 5 to 80° C.,preferably 10 to 70° C., in particular from 15 to 60° C.

The heated extract is then let down by flash evaporation to a pressurewhich corresponds at least to the pressure in the extractivedistillation zone, preferably at least to the pressure in the lowerthird of the extractive distillation zone, and is higher than thepressure in the downstream solvent recovery zone. It is critical for thepressure reduction that the vapor fraction of the extract, whichfraction forms in the flash evaporation, can be returned to theextractive distillation zone without a compression stage. Accordingly,as a rule the pressure is let down in the flash evaporation to pressureswhich are from 0.05 to 2.0, preferably from 0.1 to 1, bar above thepressure at the feed point of the vapor fraction of the extract into theextractive distillation zone. The flash evaporation is carried out, forexample, in an apparatus comprising a pressure reduction valve on anadiabatic evaporator, if required a phase separation vessel beingprovided downstream for better separation of the vapor and liquid phasesforming in the flash evaporation.

The combination of heat exchange zone for the heat exchange between theextract from the extractive distillation zone and the selective solventrecycled from the solvent recovery zone with the downstream flashevaporation can be used in one stage. However, it is also possible touse more than one such combination, for example from 2 to 4, preferably2 or 3, such combinations, advantageously connected in series. By usingmore than one of these heat exchange/flash evaporation stages andrecycling the part-streams thus obtained to different feed points of theextractive distillation zone, the required separation efficiency of theextractive distillation and the dimensions of the extractivedistillation column can be reduced. It is also possible to connect afurther heat exchange zone between the last flash evaporation zone andthe solvent recovery zone.

That vapor fraction of the extract which forms in the flash evaporationzone or zones and generally comprises from 20 to 80, preferably from 40to 70, % by weight of the hydrocarbons in the extract is returned to theextractive distillation zone. In general, the returned vapor phase ispassed into the lower third of the extractive distillation zone,preferably at the bottom of the extractive distillation zone, forexample at a point which is located roughly at the height of thelowermost column tray. In the stepwise flash evaporation, vaporfractions contained in the individual stages can be returned, separatelyor after their combination, to the extractive distillation zone.

The liquid phase of the extract from the extractive distillation zone,which phase remains after flash evaporation, is fed to a solventrecovery zone which is operated at a lower pressure than the pressure inthe flash evaporation zone. The remaining liquid extract phase is letdown to the lower pressure in the solvent recovery zone, advantageouslyby means of an intermediate pressure reduction valve. In general, thepressure in the solvent recovery zone is from 0.1 to 8, preferably from0.5 to 7, in particular from 1 to 6, bar lower than the pressure in theflash evaporation zone or zones. The solvent recovery zone may beoperated, for example, as a gas expulsion zone or as a solvent stripper.In general, heat is supplied to the solvent recovery zone, for examplevia an indirect heat exchanger using steam (reboiler).

The NMP obtained as a bottom product of the solvent recovery zone andfreed from the hydrocarbons is returned to the extraction stages (I) and(III) via the heat exchange zone in which the heat exchange with theextract from the extractive distillation zone takes place.

The product which is obtained from the solvent recovery zone containsthe hydrocarbons and is in general taken off as a top stream or as a topand side stream, passes partly or if necessary completely initiallythrough a compression stage and is fed to the extractive distillationzone after the compression. In the compression zone, the hydrocarbonstream is compressed to a pressure which corresponds at least to thepressure in the extractive distillation zone. In general, thehydrocarbon stream is compressed to pressures which are from 0.05 to 2,preferably from 0.1 to 1, bar above the pressure at the feed point ofthe vapor fraction of the extract into the extractive distillation zone.

FIG. 3 is a schematic diagram of an embodiment of the preferred variant.In this embodiment, 2 extractive distillation zones are connected inseries. The first extractive distillation zone is formed by column 1 andthe upper tapered column section 2, while the second extractivedistillation zone is formed by column 4 and the lower column section 3.The NMP is fed to the upper part of column 1 through line 5 and to theupper part of column 4 through line 6. A C₄-hydrocarbon mixture is fedto the bottom of column 1 via line 7.

The columns 2/3 and 4 are also directly connected. A gaseous part streamis removed from the column 2 and washed countercurrently with solventthrough line 6.

At the top of the column 1, a refined product which consists essentiallyof butenes and butanes is taken off through line 8.

At the top of the column 4, an essentially pure 1,3-butadiene is takenoff through line 9.

A gas stream containing essentially the other hydrocarbons and otherimpurities is removed via the side take-off of the column 10 throughline 11.

The pressure in the column section 3 is about 5 bar. The extract takenoff via line 12 is brought to 15 bar by a liquid pump 13 and then heatedat from 70?C [sic] to 125?C [sic] in heat exchanger 14 by means of theNMP taken off from gas expulsion zone 10 via line 24 and essentiallyfree of C₄-hydrocarbons. The heated extract is then passed throughpressure reduction valve 15 and let down to a pressure slightly above 5bar. While the gaseous phase formed in the phase separation tank 16 isimmediately returned through line 22 and 23 to the column 3, the liquidphase, obtained after the flash evaporation, of the extract is fedthrough line 17 to another pressure reduction valve 18 where thepressure falls to the level of pressure in the column 10, normally 1.5bar.

At the top of column 10, a hydrocarbon stream is taken off through line19 and, after compression in the compressor 20, also fed through line 23to the bottom of the column 3. It is important, for safety reasons, inthis connection that the gas stream 19 is cooled by heat exchange (notdepicted in FIG. 3) before entering the compressor 20 so that thetemperature of the gas stream after emergence from the compressor doesnot exceed 110° C. The gas stream is normally cooled to 45° C.

The NMP which is virtually free of C₄-hydrocarbons and is taken offthrough line 24 and cooled in the heat exchanger 14 is fed through line25 to the heat exchanger 2. It then passes through another heatexchanger (not depicted in FIG. 3) in which the temperature of thesolvent is adjusted to 38° C. The amount of solvent arriving throughline 26 is then divided into two part streams: line 6 leads to thecolumn 4, while line 27 terminates in the additional extractivedistillation zone (III) for separating butenes and butanes. The solventreturns from there through line 5.

In order to simplify the drawings, all the abovementioned back-washzones with the flows of liquid hydrocarbons back to columns 1 and 4 havealso been omitted. The abovementioned side column on the main column 10is also absent from FIG. 3.

EXAMPLE

Compared with FIG. 3, the diagram of the process in the example isconsiderably more extensive (FIG. 4), even though all the pumps havebeen omitted from the figure in this case too. On the other hand, allthe heat exchangers are detailed besides the columns because they arecrucially involved in the economics of the process. Likewise, all thephase separators have been included in the diagram. The numberassignment system is likewise evident from FIG. 4.

The extraction zone (I) and the desorption zone (I) are concealed behindcolumn 120. Extraction zone (III) is column 130. The desorption zone(II) is represented by the two columns 140 and 150. Extraction zone (II)is the column 100 and desorption zone (III) is column 110. In addition,the two final distillation columns 160 and 170, in which the crudebutadiene is finally brought up to specification, have been included,because they belong to the overall process.

The process has 3 incoming streams:

Stream 10: Added stream Zbc, containing butenes and butanes

Stream 40: C₄ feed

Stream 69: Condensate addition to reduce the NMP loss

Composition [% by weight] and quantity [kg/h] of stream 10:

n-Butane 26.0 i-Butane  9.5 n-Butene 42.0 trans-2-Butene 13.0cis-2-Butene  9.5 Quantity = 4200.

Composition [% by weight] and quantity [kg/h] of stream 40:

Propane 0.1 Propene 0.1 Propadiene  0.05 Propyne  0.15 n-Butane 7.3i-Butane 4.0 n-Butene 14.0  i-Butene 24.6  trans-2-Butene 4.5cis-2-Butene 3.5 1,3-Butadiene 40.0  1,2-Butadiene  0.45 Ethylacetylene0.2 Vinylacetylene  0.75 i-Pentane 0.1 3-Methyl-2-butene 0.12-Methyl-2-butene 0.1 Quantity = 15,000.

Quantity for stream 69 [kg/h]:

Quantity=1100.

Solvent required for the 3 extraction zones [kg/h] and compositionthereof 8% by weight]:(HC=hydrocarbons)

(IExtr. zone) stream 42 = 165,000 with NMP 91.63 Water  8.29 Total HC 0.08 Extr. zone (III) stream 46 = 35,000 with NMP 91.7 Water  8.3 TotalHC  1 ppm by wt. Extr. zone (II) stream 12 = 100,000 with NMP 91.63Water  8.29 Total HC  0.08 Stream 57 = 165,000 with NMP 91.7 Water  8.3Total HC  1 ppm by wt.

Temperature: generally 38° C.

In the following detailed description of the columns, the plate numbersare generally counted from the top of the column.

Operating conditions for column 100:

Number of theoretical plates = 4 + 50 (including back-wash zone) HCback-flow to plate = 1 Solvent feed to plate = 5 HC feed to plate = 42Added stream Zbc to plate = 42 Pressure at plate 1 = 4.0 bar Temperatureat plate 1 = 38.5° C. HC back-flow quantity = 5000 kg/h

Operating conditions for column 110:

Number of theoretical plates = 4 + 9 (including back-wash zone) HCback-flow to plate = 1 Extract feed to plate = 5 Pressure at plate 1 =1.526 bar Temperature at plate 1 = 6.7° C. HC back-flow quantity = 5000kg/h Energy required = 10,361 kW

Operating conditions for column 120:

Number of theoretical plates = 25 + 23 + 7 (in 2 columns) HC back-flowto plate = 1 Solvent feed to plate = 1 HC take-off at plate = 49 (to theextraction zone II) HC feed to plate = 26 Pressure at plate 1 = 4.5 barTemperature at plate 1 = 41.5° C. HC back-flow quantity = 2094 kg/h

Operating conditions for column 130:

Number of theoretical plates = 4 + 30 (including back-wash zone) HCback-flow to plate = 1 Solvent feed to plate = 5 Pressure at plate 1 =5.0 bar Temperature at plate 1 = 45.3° C. HC back-flow quantity = 2120kg/h

Operating conditions for column 140:

Number of theoretical plates = 10 Extract feed to plate = 1 HC take-offat plate = 6 (to column 150) Pressure at plate 1 = 1.52 bar Temperatureat plate 1 = 104.6° C. Temperature at plate 10 = 146.1° C. HC back-flowquantity = 2120 kg/h Energy required = 6773 kW

Operating conditions for column 150:

Number of theoretical plates = 2 Water feed to plate = 1 Extract feed toplate = 1 Pressure at plate 1 = 1.52 bar Temperature at plate 1 = 108°C.

Operating conditions for column 160:

Number of theoretical plates = 46 HC back-flow to plate = 1 Extract feedto plate = 16 Pressure at plate 1 = 7 bar Temperature at plate 1 = 46.6°C. HC back-flow quantity = 6130 kg/h Energy required = 761 kW

Operating conditions for column 170:

Number of theoretical plates = 45 HC back-flow to plate = 1 Extract feedto plate = 23 Pressure at plate 1 = 4.2 bar Temperature at plate 1 =39.3° C. HC back-flow quantity = 11197 kg/h Energy required = 1671 kW

The individual process steps are as follows:

The normally liquid C₄ hydrocarbon mixture (stream 40) is vaporized inthe heat exchanger 240 and enters the extraction zone (I) as vapor,approximately in the middle of column 120. Solvent is passed via stream42 countercurrently to the ascending gases. This results in two newstreams: a gaseous product (stream 43) containing essentially the majorquantity of propane, propene, propadiene, the butanes and the butenes,and an extract (stream 53) containing the hydrocarbons dissolved in thesolvent, comprising 1,3-butadiene and the other hydrocarbons, includingthe C₅-hydrocarbons. The 1,3-butadiene content, an importantspecification quantity for the separation requirement of the exractionzone (I), is below 100 ppm by weight.

To lower the temperature profile in the column 120, 20% by weight of thegas stream 43 are condensed in the heat exchanger 230.

A side stream leaves the column 120 via stream 51 and is washedcountercurrently with solvent (stream 46) in the extraction zone (II),which is column 130. To lower the temperature profile and, at the sametime, to reduce the NMP loss, the column has a black-flow of liquidhydrocarbons (stream 50). The solvent quantity (stream 46) is adjustedso that the specifications relating to ethylacetylene and vinylacetylenefor the later pure 1,3-butadiene are met. The decanter 320 incorporatedin the top circulation serves for partial removal of water from thecrude butadiene.

The extract from the extraction zone (I), which is stream 53, passesthrough a heat exchanger 255 and is then partially desorbed underpressure in the flash tank 410, resulting in two streams: a gaseousportion (stream 59) which is immediately recycled via stream 67 to thecolumn 120, and a liquid portion (stream 60). The temperature of thestream 60, which is still under the elevated flash pressure in the flashtank 410, is raised by 5 C in another heat exchanger 260, before it islet down by reducing the pressure in the column 140.

The extract, stream 61, is almost completely desorbed from theC₄-hydrocarbons in column 140 by input of external energy. The resultinggas, stream 62, is cooled in the heat exchanger 265 to theabovementioned 45 C and divided in the flash tank 420 into a very smallportion of liquid phase (stream 66) and the main part of gas phase(stream 64). The gas stream 64 is compressed in the compressor 500 and,after combining with the streams 59 and 66, returned as stream 67 to thelower part of the column 120.

A gas stream 68 is removed from column 140 approximately in the middle.Beside hydrocarbons, it contains water. This gas stream is washedcountercurrently with condensate in the column 150, and the gaseousproduct (stream 70) is cooled in heat exchanger 270 and, after divisioninto a gas phase and liquid phase (streams 72 and 73), discharged asproduct. The NMP content in stream 70 is about 160 ppm by weight, whichsignifies an NMP loss of 0.19 kg/h at this point.

The route taken by the almost completely desorbed solvent from thecolumn 140 passes successively through the heat exchangers 255 (heatingof the extract from column 120), 250 (reboiler for column 170), 240(vaporization of the C₄-hydrocarbon mixture) and 235 (final solventcooler to adjust the solvent temperature). It thus passes completelythrough the extraction zones (I) and (III) and the desorption zones (I)and (II).

The top product from column 120 (stream 45) and the added stream Zbc(stream 11) are fed into the lower third of the extraction zone (II),i.e. column 100. The gases are passed together with the gas streamresulting after heat exchange in the heat exchanger 215 and subsequentflash decompression in the flash tank 400 countercurrently to thesolvent (streams 12 and 57). This results in a high-purity butanefraction (stream 13) with only 0.43% by weight of butenes. Like the topproduct from column 130, the gas stream 13 is also condensed (heatexchanger 200) and then partly freed of water in the decanter 300 beforeit is returned as liquid hydrocarbon back-flow (stream 16) to column 100or is discharged as butane fraction (c), i.e. stream 17, as product.

The extract, which has already been heated by heat exchange, from theextraction zone (II), i.e. stream 20, is fed, after renewed indirectheat exchange in the heat exchanger 220 and after pressure reduction, tothe desorption zone (III), i.e. column 110. The extract is verysubstantially freed of the C₄-hydrocarbons therein by input of externalheat. The butene fraction (b) thus resulting, stream 23, is likewise ofhigh purity and contains only 1.85% by weight of butanes. Aftercondensation in the heat exchanger 225 and partial removal of water inthe decanter 225, the butene fraction is partly fed in liquid form asback-flow into column 110 (stream 26) and partly discharged as product(stream 27).

The heat exchanger 225 requires a refrigerant because of the lowcondensation temperature of the butene fraction. It is not possible toincrease the pressure level in column 110: on the one hand, the bottomtemperature of column 110 would exceed the limit of 150° C., which wouldbe equivalent to impermissible thermal stress on the solvent and, on theother hand, the desorption of the hydrocarbons would be impeded, whichcould be compensated only by additional input of external energy.

The solvent desorbed in the column 110 passes successively through theheat exchangers 220 (extract preheating), 215 (raising the temperatureof the extract for the purpose of the pressure flash), 210 (vaporizer ofthe added stream 10) and 205 (final cooler for adjusting the solventtemperature). Because of the variety of tasks, certain temperaturelevels are preset, for which reason the hydrocarbon content of stream 28cannot be indefinitely high. In the exemplary case, the concentration is800 ppm by weight. This means that it also passes completely through theabsorption and desorption zones (III).

There only remains the area with the distillation columns 160 and 170for final adjustment of the specifications of the 1,3-butadiene fraction(a). The crude butadiene (stream 80) is fed into the upper third ofcolumn 160. The gaseous top product (stream 81) is condensed and, afterpartial removal of water in a decanter 330, both recycled as liquidhydrocarbon back-flow (stream 84) to column 160, and discharged asproduct (stream 83). It should be noted that stream 81 must not exceed acertain propyne concentration for safety reasons. This limitingconcentration is pressure-dependent. Its value is 50% by volume at a toppressure of 7 bar.

The almost anhydrous bottom discharge from the column 160 is fed intocolumn 170 approximately in the middle. The mixture is fractionatedtherein to a high-purity 1,3-butadiene fraction (stream 89) and a bottomproduct (stream 91), with stream 91 representing a mixture mainly of thehydrocarbons cis-2-butene, 1,3-butadiene, 1,2-butadiene andC₅-hydrocarbons. The 1,3-butadiene yield can be influenced by the preset1,3-butadiene concentration. This is 25% by weight in the presentexample. The product specification of fraction (a), i.e. the purebutadiene, is as follows:

1,3-Butadiene = 99.6% by weight Total butenes = 0.4% by weight Propyne =10 ppm by weight 1,2-Butadiene = 50 ppm by weight Total C₄-acetylenes <5 ppm by weight Total C₅-HC < 5 ppm by weight

In conclusion, the amounts of energy [kW] exchanged for condensers andheat exchangers are listed below:

Heat exchanger 200:785 Heat exchanger 205:8991 Heat exchanger 210:482Heat exchanger 215:3127 Heat exchanger 220:4000 Heat exchanger 225:1679Heat exchanger 230:230 Heat exchanger 235:2945 Heat exchanger 240:1772Heat exchanger 245:857 Heat exchanger 250:1671 Heat exchanger 255:7264Heat exchanger 260:744 Heat exchanger 265:1729 Heat exchanger 270:572Heat exchanger 275:693 Heat exchanger 280:1757

We claim:
 1. A process for separating a C₄-hydrocarbon mixturecomprising 1,3-butadiene, butenes, butanes and other C₄-hydrocarbons,into at least 4 fractions; a) the fraction A (Fa) (Fa1) (Fa2) (Fa3)comprising 1,3-butadiene, b) the fraction B (Fb) comprising butenes, c)the fraction C (Fc) comprising butanes and d) one or more fractions D(Fd) (FdR) comprising 1,3-butadiene and the other C₄-hydrocarbons, comprising extractive distillation of said C₄-hydrocarbon mixture withN-methyl-2-pyrrolidinone or an aqueous solution ofN-methyl-2-pyrrolidinone (NMP),  wherein:
 1. the C₄-hydrocarbon mixturein gaseous form is first brought into contact with NMP in an extractionzone (I) to form an extraction solution (Ead), the ratio of NMP toC₄-hydrocarbon mixture being from 5:1 to 20:1, the 1,3-butadiene and theother C₄-hydrocarbon is being essentially completely absorbed by the NMPbut the butenes and butanes remaining essentially in the gas phase toform a gas stream (Gbc);
 2. the unabsorbed butenes and butanes (Gbc) andthe extraction solution (Ead) formed in step 1 are removed from theextraction zone (I);
 3. the extraction solution (Ead) is transferred toa desorption zone (I) at a lower pressure and/or higher temperature thanthe extraction zone (I) and 1,3-butadiene is desorbed from theextraction solution (Ead) to form the fraction A (Fa), the main part ofthe other C₄-hydrocarbons remain in the liquid phase to form anextraction solution (Ed);
 4. the extraction solution (Ed) formed in step3 and the desorbed 1,3-butadiene (Fa) are removed separately from thedesorption zone (I) and, if required, a part of the fraction A isreturned to the extraction zone (I);
 5. the extraction solution (Ed) istransferred to a second desorption zone (II) at a lower pressure and/orhigher temperature than the desorption zone (I) and having a pressureand/or temperature gradient, and the other C₄-hydrocarbon is and the1,3-butadiene still remaining therein are fractionally desorbed from theextraction solution (Ed) to form at least two separate fractions D (Fd)(FdR), with the content of the other C₄-hydrocarbons being at least 10times higher in at least one of the fractions (Fd) than in theextraction solution (Ed), based on the content of all C₄-hydrocarbons,and the content of the other C₄-hydrocarbons being correspondingly lowerin at least one of the fractions (FdR) than in the fractions D, based onthe content of all C₄-hydrocarbons, and the extraction solution (Ed)further forming NMP,
 6. the NMP, formed in the desorption zone (II) andessentially free of C₄-hydrocarbons, and the fractions (Fd) and (FdR)are removed separately from the desorption zone (II), and one or morefractions (FdR) are returned to the desorption zone (I),
 7. the gasstream (Gbc) is first brought into contact with NMP in an extractionzone (II) to form an extraction solution (Eb), the butenes beingessentially completely absorbed by the NMP but the butanes remainingessentially in the gas phase;
 8. the unabsorbed butanes (fraction C(Fc)) and the extraction solution (Eb) formed in step 7 are removed fromthe extraction zone (II);
 9. the extraction solution (Eb) is transferredto a desorption zone (III) at a lower pressure and/or higher temperaturethan the extraction zone (II) and the butenes are desorbed from theextraction solution (Eb) to form NMP essentially free ofC₄-hydrocarbons;
 10. the NMP, formed in step 9 and essentially free ofC₄-hydrocarbon is, and the desorbed butenes (fraction B (Fb)) areremoved from the desorption zone (III);
 11. the NMP formed in step 9 isrecycled to the extraction zones (I) and (II),
 12. a part of the1,3-butadienie fraction A (Fa1) which is removed from the desorptionzone (I) is returned to the extraction zone (I), and the other part(Fa2) is again brought into contact, in an extraction zone (III), withNMP which was recovered according to step 6, the ratio of NMP to thecrude butadiene fraction (Fa2) being from 1:3 to 1:7 and a part of thefraction (Fa2) and the predominant part of other C₄-hydrocarbons stillcontained as impurity in the fraction (Fa2) being absorbed by the NMP toform an extraction solution (Eax); and
 13. the unabsorbed part of thefraction (Fa2) ((Fa3)) is removed separately from the extraction zone(III), and the extraction solution (Eax) is returned to the extractionzone (I).
 2. A process as claimed in claim 1, wherein the NMP and theC₄-hydrocarbon mixture are fed into the extraction zone (I) in a ratioby weight of from 5:1 to 20:1.
 3. A process as claimed in claim 1,wherein the C₄-hydrocarbon mixture contains from 10 to 80% by weight of1,3-butadiene; from 10 to 60% by weight of butenes; from 5 to 40% byweight of butanes; and from 0.1 to 5% by weight of otherC₄-hydrocarbons.
 4. A process as claimed in Claim 1, wherein theextraction zone (I) is in the form of 2 columns connected to oneanother.
 5. A process as claimed in claim 1, wherein the extractionzones (I), (II) and (III) are in the form of columns and the fractionsand C₄-hydrocarbon mixture are passed countercurrently to the NMPthrough said columns.
 6. A process as claimed in claim 1, wherein thedesorption described in steps 3 and 5 of claim 1 is effected byincreasing the temperature in the desorption zone (I) relative to thatin the extraction zone (I) and increasing the temperature in thedesorption zone (II) relative to that in the desorption zone (I) bysupplying the heat of the NMP, which has been removed according to step11 of claim 1 from the desorption zone (II), to the desorption zone (I)by indirect heat exchange in a heat exchange zone.
 7. A process asclaimed in claim 1, wherein another hydrocarbon stream (added streamZbc), the latter essentially consisting of a mixture of butanes andbutenes, is added to the gas stream formed in step
 1. 8. A process asclaimed in claim 1, wherein the temperature is increased in thedesorption zones (II) and (III) for the purpose of complete desorptionof the NMP from hydrocarbons by indirect heat exchange with steam.
 9. Aprocess as claimed in claim 1, wherein the back gas required forseparating the fractions (b) and (c) is obtained in the extraction zone(II) by partial desorption of the extraction solution (Eb), the requiredincrease in temperature originating, by indirect heat exchange, from theNMP freed of C₄-hydrocarbons in the desorption zone (III).